The field of the invention is plastic part fabricating machines, and the invention relates more specifically to blow molding machines. Such machines are commonly used to fabricate various hollow shapes and especially bottles. In blow molding, a tube or parison is formed by an extruder and as the tube moves downwardly to its lowermost position, two mold halves are moved toward one another and against the extruded parison. After the mold halves have closed, air is injected through the upper end of the parison to force the parison against the inner walls of the mold.
In order to reduce weight and decrease costs, there is a desire to make thin-walled, light-weight articles. For some applications, such as milk bottles, it is also desired to form a handle as a portion of the bottle to permit carrying the bottle with one hand and to facilitate pouring. Because of the thin wall thickness, the parison often does not maintain its shape and occasionally the handle will not be completely formed and a defective bottle results.
In order to help maintain the shape of the parison, air is often blown outwardly against the upper edge of the parison. This air infringes against the parison near the mandrel and then moves downwardly and out at the lower edge of the parison. For thin-walled containers of relatively small inside diameters such as one-gallon bottles, this downward flow of air may actually have an adverse effect on parison shape because of the Bernoulli effect. The rapid flow of air along the inner surface of the parison as it extends downwardly decreases the pressure along the inside surface of the lower part of the parison. Ironically, the greater the flow of air inside the parison, the more the sides of the parison tend to move tegether at the lower portion.
It has been known to blow air upwardly into large heavy wall industrial containers or large containers using large tooling. The usual steps taken to try to solve this problem included increasing air flow at the top of the parison and aiming the air flow upwardly and outwardly. Such steps were only partially effective and a better system is needed.
Another problem with high volume blow molding operations is the amount of operator time required to make adjustments to keep the parison straight. One operator is typically needed for two machines.
A further problem exists when the extrusion pressure reduces near the end of the extrusion cycle referred to as "shot pressure tail off." It is the latter part of the extrusion which determines the configuration of the portion of the container including the handle and thus "shot pressure tail off" has an adverse effect on container quality. This is especially true when the polymer used exhibits a substantially lower amount of die swell. This problem is also particularly acute for one-half gallon and one gallon single service containers. Another problem is referred to as "tail folds" which results when the lower portion of the parison folds inwardly. When a folded parison is moved outwardly against the mold, a fold is formed in the molded part resulting in a reject.